Field of the Invention
This invention lies in the field of telephone apparatus. More particularly, it concerns electronic circuitry that can be connected to a receiver station for determining when the caller is one of a selected group of persons who have been given a private code number in order to make contact with the receiving station.
No change is required in the conventional telephone instrument which is still used in its proper way but is connected to the telephone line and to the central station in shunt with an electronic control circuit.